1. Field of the Invention
The present invention relates to an image forming apparatus that employs an electrophotographic method.
2. Description of the Related Art
Conventionally, multi-color image forming apparatuses that employ a so-called multipath system (also called a “four-path system”) are known in which toner images of the four colors yellow, cyan, magenta and black, are sequentially superposed on a transferring member. In such multi-color image forming apparatuses, if the positions in which images of a plurality of colors are superposed are relatively displaced among the colors, color misalignment occurs. For this reason, it is necessary to perform positional alignment among the toner images of the respective colors to be superposed on the member as accurately as possible.
On the other hand, there is a demand for reducing the user waiting time from inputting a print job to the first printout, improving productivity and the like in multi-color image forming apparatuses from the viewpoint of usability. This applies also to the above-described multipath printers. In the multipath system printers, a mode is already known in which processes that have been performed in series in conventional printers are performed in parallel. For example, it is conceivable to change execution of secondary transfer of the toner image of the previous page and primary transfer of the toner image of the next page from conventional serial execution to parallel execution.
In such a mode, for example, cases in which cleaning or the like is performed and is not performed in parallel with the primary transfer operation for the last color of a plurality of colors subjected to primary transfer both occur. In this case, if a mechanical shock occurs due to members coming into contact and separating in relation to cleaning or the like, the condition of a load applied to the image forming units differs for each image forming color. As a result, color misalignment occurs.
In Japanese Patent Laid-Open No. 2000-66475, a registration correction control technique is proposed in which in a multipath system electrophotographic printer, consideration is given to displacement of the latent image formation position or the primary transfer position depending on whether or not there is a mechanical shock at the time of primary transfer of a toner image. Specifically, Japanese Patent Laid-Open No 2000-66475 proposes a technique for correcting the image-writing start timing in the case where there is a mechanical shock.
The above-described mechanical shock may be caused by random factors or the like, and it is difficult to accurately predict the amount of color misalignment that will actually occur. That is, it is difficult to predict the amount of color misalignment caused by a mechanical shock and correct that color misalignment, and thus it is desired to further improve prediction accuracy.
On the other hand, as another issue, in recent years, further reduction in the size, cost and the like of image forming apparatuses has been sought. With the reduction in size, cost or the like, innovative ways of correcting color misalignment are sought more strongly than ever.
Note that it is possible to avoid color misalignment due to a mechanical shock by serially performing image formation of a certain page, an operation causing a mechanical shock and image formation of the next page in order to perform image formation while avoiding the above-described mechanical shock. However, this results in deterioration in the productivity of image formation, that is, deterioration in usability.